A direct conversion technique is very suitable for high data rate applications, and therefore the technique is now widely used, such as a direct conversion receiver. In the direct conversion receiver, a mixer plays a very important role. The mixer converts an RF signal into an intermediate frequency (IF) or baseband signal. However, the mixer may induce noise into the receiver, so that the noise figure of the receiver is degraded. In addition, linearity of the mixer affects the dynamic range of the receiver. Therefore, various mixer structures are proposed in order to achieve desirable noise figure as well as linearity.
FIG. 1 shows a structure of a conventional Gilber mixer 100. The mixer 100 includes an inputting stage having a switch pair (transistors 112, 114) and a transconductance circuit comprising degeneration resistors 126, 128 and a current source 130, a switching stage having a switch quad (transistors 142, 144, 146 and 148), and a load stage having resistors 156 and 158. In this example, the transistors are implemented by CMOS transistors.
In the inputting stage, the gates of the transistors 112 and 114 are connected to an RF signal source 106. One terminal of the degeneration resistor 126 is connected with the source of the transistor 112, and one terminal of the degeneration resistor 128 is connected with the source of the transistor 114. The other terminals of the degeneration resistors 126 and 128 are connected together and connected to ground via the current source 130. The drains of the transistors 112 and 114 are connected to the switching stage.
In the switching stage, the gates of the pair of transistors 142, 144, and the gates of the pair of transistors 146, 148 are connected to a local oscillation signal source 108. The transistors 142 and 144 have the sources thereof connected together and connected with the drain of the transistor 112. The transistors 146 and 148 have the sources thereof connected together and connected with the drain of the transistor 114. The drains of the transistors 142 and 146 are connected together as an output port and connected to the resistor 156. The drains of the transistors 144 and 148 are connected together as the other output port and connected to the resistor 158.
An RF signal is input to the inputting stage of the mixer 100 from the RF signal source 106, and is down-converted by the switching stage with a local oscillation signal provided by the local oscillation signal source 108, and then the down-converted signal is outputted via the output ports.
In this structure, the degeneration resistors 126 and 128 are used to improve the linearity of the mixer 100. The current passed by the current source 130 is divided between the transistors 112 and 114. The degeneration resistors 126 and 128 increase a maximum useful input voltage for the mixer and set an effective transconductance for the mixer.
However, such a structure has a disadvantage of a conflict between noise figure and linearity. If the current and the resistance of the degeneration resistors are decreased, the noise is reduced but the distortion is increased. Conversely, if the current and the resistance are increased, the distortion is reduced but the noise is increased.
Therefore, there is a need for a mixer which may provide qualified linearity, in the meanwhile, noise is depressed.